A cylindrical, rotating electrical machine consists of a field coil that produces a magnetic field and an armature. The armature consists of a series of current loops in the form of a coil that links the magnetic flux produced by the field coil. Relative rotational motion between the armature and the field coil then produces a generated voltage in the armature due to the rate of change of flux linkage in the armature coils. The power of the machine is related to the generated voltage that is produced in the armature. Rotating machines are usually AC (synchronous) or DC types.
For example, in a “synchronous machine” such as a conventional AC generator, a rotating field coil is situated within the aperture of a stationary armature, which may contain multi-phased coils. The rotating field interacts with the stationary armature to produce a sinusoidal-varying voltage in the armature coils.
Another example of a rotating machine is a conventional DC-type machine in which a rotating armature is located in the aperture of a stationary field coil. The armature coils in this case also experience a generated alternating voltage, but the armature current is rectified by commutation or by external rectification to produce a voltage that is substantially DC.
It is seen that the synchronous machine derives its armature flux linkage from the field external to the field coil, while the “DC type” machine derives its armature flux linkage from the field inside the aperture of the field coil. This amounts to “wasted flux” in both cases because the field on the inside of the field coil of the synchronous machine is not linked to the coils of the armature and the field on the outside of the field coil in the “DC type” machine is not linked to the coils of the armature.
In the case of DC machines, there are two configurations that have been used that include a superconducting field coil with a non-superconducting armature or rotor. One is the homopolar machine with a superconducting field coil and a high-current resistive rotor that connects to an external circuit. In this case there is only one effective current path in the rotor and thus the machine can only produce power at low voltage and high current. However, the current is a constant voltage DC. The second type of DC machine can be considered as a rectified AC machine that uses, for example, a three-phase winding for the armature coils, coupled with a full-wave three-phase rectifier.
A known example of the homopolar motor is described by Michael J. Superczynski, Jr., and Donald J. Waltman in IEEE Transactions on Applied Superconductivity, Vol. 7, No. 2, June 1997, “Homopolar Motor with High Temperature Superconductor Field Windings”. The machine described in Sperczynski uses a rotor that is supplied by 30,000 A of current in order to produce the desired power. This high current is necessary because the rotor provides only a single current path through the magnetic field, and the output power is proportional to the total current, the field strength, and the length of the current path in the rotor. Thus, the output of the machine of a fixed size and magnetic field can only be increased by increasing the current passing through the rotor.
An externally excited DC machine, using the principle of rectified AC with a superconducting field coil and a normal-conducting armature is disclosed in U.S. Pat. No. 5,032,748, Jul. 16, 1991, to Sakuraba et al.
Lewis and LeFlem describe a superconducting electrical machine of the synchronous type in patent application document US 2008/0161189A1, which was published Jul. 3, 2008. The device described in Lewis is a synchronous electrical machine with a rotating field coil that comprises two concentric cylindrical coils and a stationary single armature coil that is situated in the gap between the two field coils. This split field coil configuration allows the armature to link the same amount of flux that can be linked from an un-dual field coil that must operate with a higher field than either of the dual field coils. In this case, the operating margin of the field coils is increased because they can now run at a lower current that does not exceed the critical current density in the superconductor. This approach is designed to allow more choices for superconductor, but this configuration does not significantly change the power density of the machine since the armature does not link with all of the field flux.
Caroon describes a prior art configuration in U.S. Pat. No. 7,400,077. This Caroon machine has a central rotatable axle, a first ‘field armature’ disposed around and attached to the axle, a second ‘field armature’ that is disposed around the first field armature and also attached to the axle, and a stationary ‘electromagnetic member’ that is disposed between the first and second field armatures and is attached to the machine housing. The ‘field armatures’ each contain a plurality of field magnets while the electromagnetic member includes a plurality of electromagnets. The arrangements of field magnets and electromagnets on the rotors and stator are much more complex than those described in the present invention.
Minagawa describes a prior art configuration in U.S. Pat. No. 6,710,492. This Minagawa machine claims a stator with a plurality of coils that are supplied with a polyphase AC current to cause two rotors with different arrangements of magnets and coils to move independently of each other. In the claims, the two rotors are coaxial and concentric in some configurations and undefined in other claims. The position of the stator is also not defined in any of these claims. The description of the machine in the text shows a triple layer structure with the stator positioned between these two rotor coils but in the final paragraphs it is stated that “it is possible to apply the invention to a motor/generator disposing two rotors coaxially.” The arrangements of magnets and coils on the rotors and stator are much more complex than those described in the present invention.
Smith describes a prior art configuration in U.S. Pat. No. 3,742,265 This machine claims various arrangements of three concentric and coaxial cylindrical elements, of which one is a superconducting field winding and the other two are normal conducting armatures. Some of the claims describe placement of the field winding in the center between the two armature windings, however, the stated objective in claim 1 is not the same as the present invention, which operates differently to produce a different functional result. The present invention can also use all superconducting elements or field coils, and the present invention can also use normal conductors in armatures.